Production of pyrotechnic delay composition

ABSTRACT

A process for producing a pyrotechnic delay composition includes admixing an oxidizer, a fuel, a surfactant and a liquid, to form a paste or slurry. The paste or slurry is dried to remove the liquid and to obtain a solid product which, if necessary, is rendered into particulate form. Optionally, the solid particulate product is classified to obtain a pyrotechnic delay composition in particulate form.

THIS INVENTION relates to the production of a pyrotechnic delay composition of the type used, for example, in delay elements employed for the delayed initiation of explosives. More particularly, the invention relates to a process for the production of such composition, and to a pyrotechnic delay composition.

According to a first aspect of the invention, there is provided a process for producing a pyrotechnic delay composition, the process including

-   -   admixing an oxidizer, a fuel, a surfactant and a liquid, to form         a paste or slurry;     -   drying the paste or slurry to remove the liquid and to obtain a         solid product;     -   if necessary, rendering the solid product into particulate form;         and     -   optionally, classifying the solid particulate product, to obtain         a pyrotechnic delay composition in particulate form.

The pyrotechnic delay composition, when used in the manufacture of delay elements employed for the delayed initiation of explosives, provides the delay elements with a desired burning rate.

The liquid may be an organic liquid such as a solvent. Instead the liquid may be water.

Typically, in the manufacture of pyrotechnic delay compositions of the kind in question by a process of the type to which the present invention relates, the exact oxidizer and fuel constituents, their particle sizes and their proportions in the paste or slurry mixture, are selected in accordance with practical and economic considerations, bearing in mind safety and the intended use of the delay compositions.

The oxidizer may be in solid particulate form. The oxidizer may comprise red lead, barium sulphate and/or potassium perchlorate. The oxidizer may comprise 40-90% by mass of the composition.

The fuel may also be in solid particulate form. The fuel may comprise silicon, zinc and/or magnesium. The fuel may comprise 5-60% by mass of the composition.

The surfactant or surface active agent may be in particulate form, and may be a wetting agent and/or a rheology modifier and/or a binder (binding agent). The surfactant may be selected from the so-called non-ionic surfactants, anionic surfactants and cationic surfactants. The surfactant may thus comprise an acrylic ester, a styrene polymer, and/or an acylic copolymer, which are all wetting agents; and/or a polyethelene glycol, a powdered smectite clay, carboxymethyl cellulose, polyvinyl alcohol and/or polyvinyl pyrrolidone which are rheology modifiers or thickeners. The surfactant may be in the form of an aqueous dispersion when admixed with the oxidizer and the fuel. The surfactant may comprise 0.25% to 4%, by mass, of the paste or slurry before drying, i.e. on a wet basis. Typically the surfactant may comprise 0.1% to 2%, by mass, of the paste or slurry.

In particular, at least one of the surfactants used may function, in the paste or slurry, as a rheology modifier such as a thickening agent. Such rheology modifiers will typically be selected for their ability, not only to alter or modify the burning rate of delay elements made from the delay compositions in question, but also to resist so-called sedimentation or separation of the constituents of the paste or slurry, after formulation thereof and before the drying.

The surfactant may comprise 0.1-2% by mass of the composition. Thus, the composition may comprise

oxidizer 40-90% fuel 5-60% surfactant 0.1-2%

It is expected that the mixing of the constituents to formulate the paste or slurry will be carried out in a more or less conventional manner, using conventional plant or equipment such as Z blade or high shear mixers, the surfactant or surfactants being added at a convenient time and in the required proportions, to become homogeneously dispersed in the paste or slurry. It is contemplated that two or more surfactants may be employed together, suitable surfactant mixtures thus being used when desired.

Likewise, drying of the paste or slurry, rendering of the solid product into particulate form, and classifying the solid particulate product, may be carried out in more-or-less conventional manner. For example, to obtain the particles, drying of the paste or slurry in an oven, and hand granulation (eg forcing the material through a screen) may be employed. However, in another embodiment of the invention, spray drying of the slurry, whereby the slurry is pumped through an orifice in a two fluid nozzle while simultaneously passing compressed air through the nozzle, to atomize the slurry into droplets, with the resultant droplets being dried by means of hot air, thereby to obtain more-or-less spherical product particles, may be employed.

Traditionally, burning rates of delay elements made from delay compositions are controlled or modified by altering the particle size and/or the oxidizer:fuel mass ratio in the mixture. However, there is a lower particle size limit, below which control of burning rate by changing particle size becomes problematic and unreliable; and, similarly, the oxidizer:fuel ratio has upper and lower limits, beyond which the compositions can no longer reliably sustain combustion. It is thus a feature of the present invention that its technique for controlling burning rates avoids or reduces these problems.

The invention extends also to a pyrotechnic delay composition when produced in accordance with the first aspect of the invention.

According to a second aspect there is provided a pyrotechnic delay composition, which is in solid, particulate form, with the particles comprising an oxidizer, a fuel and a surfactant.

The oxidizer, fuel and surfactant may be as hereinbefore described.

As hereinbefore described, the delay composition may comprise, on a mass basis,

oxidizer 40-90%; fuel 5-60%; and surfactant  0.1-2%.

The invention will now be described, by way of non-limiting illustrative example, with reference to the following examples.

EXAMPLE 1

Pyrotechnic delay compositions, in accordance with the invention, were formulated having the following compositions (proportions expressed as % by mass):

Constituent A B barium sulphate particles (d50 approx. 3 μm) (oxidizer) 38 38 red lead particles (d50 approx. 3 μm) (oxidizer) 54 53 silicon particles (d50 approx. 3 μm) (fuel) 7 7 Solsperse 20000 (100% (ie undiluted) active polymeric 1 2 dispersant) (surfactant) 100% 100%

The pyrotechnic delay compositions were produced as follows: The oxidizers, fuel and surfactant were mixed by hand or by using a high shear mixer, together with sufficient water, to obtain a slurry; the slurry was then oven dried to a consistency that allowed hand granulation by pushing the dried product through a 1 mm screen; thereafter, the resultant granules were further oven dried to reduce the water content to less than 1%, by mass. Solsperse 20000 is manufactured by Avecia, and distributed in South Africa by Lubrisol.

The composition comprising 1% Solsperse 20000 and 54% red lead had a burning rate of 19 milliseconds/mm in 3.6 mm inner diameter aluminum tubes, whereas, for the composition comprising 2% Solsperse 20000 and 53% red lead the burning rate decreased to a value of 80 milliseconds/mm.

EXAMPLE 2

Pyrotechnic delay compositions, in accordance with the invention, were formulated having the following compositions (proportions expressed as % by mass):

Constituent A B barium sulphate particles (d50 approx. 3 μm) (oxidizer) 38 38 red lead particles (d50 approx. 3 μm) (oxidizer) 54 51 silicon particles (d50 approx. 3 μm) (fuel) 7 7 Acrinol 296D (Aqueous dispersion of acrylic ester/styrene 1 4 polymer) (surfactant) 100% 100%

The pyrotechnic delay compositions were produced in the same manner as in Example 1. The Acrinol 296D was obtained from BASF SA (Pty) Limited of 852 1 6th Road, Midrand, Gauteng, South Africa.

The composition comprising 1 % Acrinol 296D and 54% red lead had a burning rate of 22 milliseconds/mm in 3.6 mm inner diameter aluminum tubes, whereas, for the composition comprising 4% Acrinol 296D and 51% red lead the burning rate decreased to a value of 55 milliseconds/mm.

EXAMPLE 3

A pyrotechnic time delay composition was prepared (as hereinafter described) having the following composition in terms of solids on a dry basis (proportions expressed as % by mass):

Constituent % red lead particles (d50 approx. 3 μm) (oxidizer) 38.25 barium sulphate particles (d50 approx. 3 μm) (oxidizer) 54.25 silicon particles (d50 approx. 3 μm) (fuel) 7 smectite clay particles (BENTONE ®EW) (rheology 0.5% modifier/thickener) 100%

All four the dry particulate constituents were homogeneously mixed with water to form a slurry in which the water formed 50% by mass, with the solids thus forming 50%. The BENTONE®EW was obtained from Carst & Walker (Pty) Limited of Zenith House, 12 Sherborne Road, Parktown, Johannesburg, South Africa. The slurry was pumped, at a low pressure of 10-100kPa, along a feed line and through a 1.5mm or 2 mm diameter orifice of a centrally positioned (in a spray-drying chamber) upwardly directed two fluid spray nozzle (together with compressed air), thereby being atomized and thus formed into droplets, while low pressure air at a temperature of 210° C. was fed into the chamber via filters and a heater, by a fan, to dry the droplets. Spray-drying thus took place in the chamber to form more or less spherical dried particles of more-or-less homogeneous composition. These particles had a moisture content of about 0.1% by mass and remained in the chamber for a period of 1-40 seconds. The dried particles were collected through a solids outlet of the camber. The drying air, which issued from the chamber at 80° C. via an air outlet, was cleaned by passing it through a cyclone, a primary bag filter, and two secondary filters. Dried particles were withdrawn from an outlet of the cyclone. Dried fines were removed from the bag filter.

The dried product was found to comprise acceptably low proportions of both oversize and undersize particles which could be used, without additional classifying, as a pyrotechnic time delay composition in the manufacture of pyrotechnic time delay elements.

EXAMPLE 4

A pyrotechnic time delay composition was prepared (as hereinafter described) having the following composition in terms of solids on a dry basis (proportions expressed as % by mass):

Constituent % barium sulphate particles (d50 approx. 3 μm) (oxidizer) 54.75 silicon particles (d50 approx. 3 μm) (fuel) 44.75 smectite clay particles (BENTONE ®EW) (rheology 0.5 modifier/thickener) 100%

The pyrotechnic time delay composition was produced in the same manner as that of Example 3.

As was the case in Example 3, the dried product was found to comprise acceptably low proportions of both oversize and undersize particles which could be used, without additional classifying, as a pyrotechnic delay composition in the manufacture of pyrotechnic time delay elements.

Conventionally, in pyrotechnic time delay compositions, an oxidizer such as red lead is used to import sensitivity into the composition, particularly for compositions having a slow burning rate, e.g. about 210 ms/mm. It has thus unexpectedly been found that, by employing a surfactant in accordance with the invention in the production of a pyrotechnic time delay composition, it is possible to eliminate the use of red lead, which is desirable due to the hazardous nature of red lead, while still obtaining acceptable burning rates.

Furthermore, it is important that the surfactant used is such that little or no gas is generated by the surfactant when the composition burns. Gas generated by the burning surfactant could lead to malfunctioning of a delay element incorporating the composition. 

1. A process for producing a pyrotechnic delay composition, the process including admixing an oxidizer, a fuel, a liquid and a rheology modifier having the ability to alter or modify the burning rate of a delay element made from the delay composition, to form a paste or slurry; drying the paste or slurry to remove the liquid and to obtain a solid product; if necessary, rendering the solid product into particulate form; and optionally, classifying the solid particulate product, to obtain a pyrotechnic delay composition in particulate form.
 2. The process according to claim 1, wherein the rheology modifier is in particulate form.
 3. The process according to claim 2, wherein the rheology modifier comprises a polyethylene glycol, polyvinyl alcohol, polyvinyl pyrrolidone and/or a powdered smectite clay.
 4. The process according to claim 2, wherein the rheology modifier, when admixed with the other solid constituents, is in the form of an aqueous dispersion.
 5. The process according to claim 1, wherein the rheology modifier comprises 0.25% to 4%, by mass, of the paste or slurry.
 6. The process according to claim 5, wherein the rheology modifier comprises 0.1% to 2%, by mass, of the liquid-containing paste or slurry.
 7. The process according to claim 1, wherein the liquid is an organic solvent.
 8. The process according to claim 1, wherein the liquid is water.
 9. The process according to claim 1, wherein the oxidizer comprises red lead, barium sulphate and/or potassium perchlorate.
 10. The process according to claim 1, wherein the fuel comprises silicon, zinc, and/or magnesium.
 11. The process according to claim 1, wherein the composition comprises, on a mass basis, oxidizer 40-90% fuel 5-60% rheology modifier 0.1-2%
 12. A pyrotechnic delay composition, which is in solid particulate form, with the particles comprising an oxidizer, a fuel and a rheology modifier having the ability to alter or modify the burning rate of a delay element made from the delay composition.
 13. A pyrotechnic delay composition according to claim 12, which includes, in addition to the rheology modifier, another surfactant.
 14. A pyrotechnic delay composition according to claim 12, wherein the rheology modifier comprises a polyethylene glycol, polyvinyl alcohol, polyvinyl pyrrolidone and/or a powdered smectite clay.
 15. A pyrotechnic delay composition according to claim 12, wherein the oxidizer comprises red lead, barium sulphate and/or potassium perchlorate.
 16. A pyrotechnic delay composition according to claim 12, wherein the fuel comprises silicon, zinc, and/or magnesium.
 17. A pyrotechnic delay composition according to claim 12, wherein the composition comprises, on a mass basis, oxidizer 40-90% fuel 5-60% rheology modifier 0.1-2%
 18. A pyrotechnic delay composition according to claim 12, which contains no red lead.
 19. The process according to claim 1, which includes admixing with the oxidizer and the fuel, another surfactant in addition to the rheology modifier.
 20. The process according to claim 1, wherein the pyrotechnic delay composition contains no red lead. 